Heracross

Great League: 2 / 5

Heracross can use its unique typing, good stats, and strong moves to act as a niche closer. It's particularly effective against Dark types and can provide solid counter-coverage for Psychic and Ghost-types. The problem here is the fact that its restricted CP does it no favors, and common Pokemon with effective counter-coverage are running around the tier, such as Altaria and Deoxys-D, though Psychic-types in particular also need to be weary of STAB Megahorn. Just note that Heracross fares particularly poorly against Ghost-types, and needs to run very quickly if its opponent has any chance of packing Flying-type moves in particular.

Ultra League: 3 / 5

Heracross is actually fairly good in the Ultra league for the reasons outlined above in conjunction with its substantially less restricted CP. It's a threat to top Steel pokemon, but also destroyed by both Giratina forms. It still acts best as a closer and needs to be paired with a pokemon that can cover its weaknesses, but it can definitely punch holes in the opponent's team if played right.

Master League: 3.5 / 5

Heracross' performance stays largely the same in the Master League, acting as a closer with typing to counter top threats. It's particularly good at dealing with Dialga, but it's rendered downright trivial in the face of many other threats such as Togekiss and both forms of Giratina. At this stage, Heracross will tend to either perform remarkably well or be little more than dead weight.